1. Field of the Invention
The present invention relates to a conductivity modulation MOSFET used in applications such as a power switching device and a method of making the same.
2. Discussion of the Related Art
A conductivity modulation MOSFET, also called an insulated gate bipolar transistor (IGBT), is constructed such that the drain region of a vertical power MOSFET is used as a collector layer whose conductivity type is opposite to that of the source region.
The structure of the conventional conductivity modulation MOSFET will be described using an n-channel IGBT, however, a p-channel IGBT may be used as well. In FIG. 2, an n.sup.+ buffer layer 2 of medium impurity concentration is layered over a p.sup.+ collector layer 1 and an n.sup.- high resistance layer 3 is further layered over the buffer layer 2. P-base diffusion regions 4 are selectively formed on the surface region of the high resistance layer 3. Further, n.sup.+ emitter diffusion regions 5 are selectively formed on the surface regions of the base diffusion regions 4.
Gate electrodes 6 made of polycrystalline silicon are formed above the surface of the structure, with gate oxide films 7 being interlayered therebetween. An emitter electrode 9 contacts both the base diffusion regions 4 and the emitter diffusion regions 5. The emitter electrode 9 is insulated from the gate electrodes 6 by insulating films 8. A collector electrode 10 is in contact with the collector layer 1. The gate electrode 6 is connected to a gate terminal G; the emitter electrode 9 is connected to an emitter terminal E; and the collector electrode 10 is connected to a collector terminal C.
In the IGBT thus constructed, if a voltage, positive with respect to the emitter electrode 9, is applied to the gate electrode 6, an inversion layer is formed in a channel region 11 of the base region 4 which is located between the emitter diffusion region 5 and the n high resistance layer 3. Electrons from the emitter diffusion region 5 pass through the channel to be injected into the high resistance layer 3. Consequently, the potential of the high resistance layer drops to be equal to that of the emitter electrode 9, and the junction between the p.sup.+ collector layer 1 and the n.sup.- layer 3 is forwardly biased. Under this condition, hole current is injected from the p.sup.+ collector layer 1 into the high resistance layer through the n.sup.+ layer 2. The injection of the minority carriers causes the conductivity modulation effect in the structure, and hence, reduces the resistance of the n.sup.- layer 3. Accordingly, the resultant conduction modulation MOSFET has an on-resistance much smaller than that of the ordinary MOSFET.
The turn-off of the MOSFET of this type is achieved by reducing the MOS gate voltage to 0 V or a negative voltage. The turn-off state terminates at the instant the electrons and holes accumulated in the n.sup.- high resistance layer 3 disappear completely. The holes accumulated in the n.sup.- high resistance layer 3 reach the emitter electrode 9 via the p-base region 4. The electrons are either combined with the holes within the n.sup.- high resistance layer 3 or attracted to the p.sup.+ collector layer 1.
The turn-off phenomenon of the IGBT resembles that of the open-base bipolar transistor and takes a relatively long duration of time. In particular, the turn-off loss caused by the exponentially decreasing fall current, called a tail current, is very large, and is one of the major defects of the conductivity modulation MOSFET. The insertion or use of the n.sup.+ buffer layer 2, which functions to reduce the hole injection efficiency, is one of the measures taken for this defect, as described in "Extended Abstract IEDM 83", p. 79 (1983). To reduce the lifetime of the n.sup.- high resistance layer, there has been proposed various lifetime control methods, as described in IEEE Trans. Electron Devices, ED-31, p. 1790 (1945). To eliminate the electrons which have accumulated during the turn-off period, the electrons are pulled to the collector electrode by a collector short IGBT, in which the collector electrode 10 is shorted to the high resistance layer 3 (FIG. 2), as proposed in PCIM '88 Proc. p. 189 (1988). In the proposal, the short-circuiting of the collector electrode 10 with the high resistance layer 3 reduces the quantity of the holes injected and provides a path connecting to the collector electrode 10. Accordingly, the carriers are rapidly pulled out to reduce the turn-off time.
However, since the proposal of the insertion of the buffer layer reduces the quantity of the injected holes, the conductivity modulation effect is degraded and the on voltage is increased. A proposal of the lifetime control is also relevant with the increase of the on-resistance to reduce the lifetime of the thick high resistance layer.
FIG. 3 illustrates an exemplar of a collector short IGBT having a breakdown voltage of about 1200 V. In the illustration showing the structure and dimensions of the IGBT, the n.sup.- high resistance layer 3 and the collector electrode 10 are shorted by the n.sup.+ layer 2 adjoining the p.sup.+ layer 1. The p.sup.+ layer 1 and the n.sup.+ layer 2 are formed by diffusing an impurity material into an n.sup.- substrate of 200 um thick to the depth of approximately 30 uM. In fabricating the IGBT of FIG. 2 so as to have a breakdown voltage of about 1200 V, an 100 um thick epitaxial layer, which consists of the n.sup.+ layer 2 and the n.sup.- layer 3, is grown on the p.sup.+ substrate layer 10 of 500 um thick. To realize such a thin n.sup.- layer 3 in the structure of FIG. 3, a thin substrate must be used. However, the thickness of the substrate must be at least 200 um because of the geometrical requirements for the substrate in the diffusion process. Accordingly, the movement of the carriers accumulated in the thick n.sup.- high resistance layer 3 becomes slow, and the tail current becomes long. To improve these defects, a lifetime control may be employed. The reduction of the lifetime of the thick n.sup.- high resistance layer 3 leads to a considerable increase of the on-voltage.
Turn-off waveforms of the IGBT for different loads coupled therewith are illustrated in FIGS. 4(a)-4(c). FIG. 4(a) shows a turn-off waveform for a resistive (R) load; FIG. 4(b) shows a turn-off waveform for an inductance (L) load; and FIG. 4(c) shows a turn-off waveform for a resonance-circuit load. In the turn-off waveform shown in FIG. 4(b), which is generated when the L load is turned off, the collector-emitter voltage varies little in the tail portion of the waveform. A high voltage is applied to the collector. Accordingly, the collector electrode rapidly pulls the electrons from the high resistance layer and the fall current decreases relatively sharply. In the turn-off waveform of FIG. 4(c) for the resonance-circuit load, when the decreasing current approaches its termination, the collector-emitter voltage starts to increase rapidly. The rapidly increasing change, dv/dt, produces Cdv/dt, viz., a junction capacitance displacement current, and electrons are discharged from a growing depletion layer and attracted to the collector electrode. The holes accumulated in the n.sup.- high resistance layer are attracted to the depletion layer. The electron current discharged from the depletion layer, as a base current, triggers the injection of the holes from the p.sup.+ collector layer. Those actions, in conjunction with the Cdv/dt current, generate a relatively large current I.sub.f as shown, resulting in a large turn-off loss, where the turn-off loss is defined by ##EQU1## dt after the turn-off. To reduce the turn-off loss, it is necessary to reduce the hole current based on the Cdv/dt. In other words, it is necessary to eliminate the holes as rapidly as possible. To do this, the formation of a conduction path of the electron current to the collector and the reduction of lifetime of the high resistance layer are essential. In the collector short IGBT having the conduction path of the electron current to the collector electrode, it is impossible to reduce the lifetime because the high resistance layer is too thick.
Similar problems also exist in the p-channel IGBT where the conductivity type and hence the carrier moving directions are opposite to the n-channel IGBT.